In electronic photography, CMOS imagers can possess higher levels of imager noise than their predecessors CCD imagers. This noise can be of the form of temporal variation and fixed pattern noise. Fixed pattern noise includes Dark Fixed Pattern Noise, which is the pixel to pixel variation in response offset and Pixel Response Non-Uniformity which is the pixel to pixel variation in response to a given exposure.
Noise reduction is practiced in the art using dark fixed pattern subtraction as in U.S. Pat. No. 6,424,375. Here an electronic circuit is used to remove dark fixed pattern noise by electronically adjusting pixel responses to align them to an aim response. U.S. Pat. No. 6,418,241 discloses a system in which column biases are corrected after measuring the average of each column and adjusting each column to some aim bias. The Canon D30 digital camera also apparently performs dark captures with the shutter closed in order to obtain an estimate of the sensor's dark frame response. Also proposed is to use a dark frame capture and a flat field capture before every image capture. This is a practical impossibility in typical picture taking. There is thus a need for improved technique for minimizing fixed pattern noise.